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(11) | EP 0 137 578 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | A method of fracturing a subterranean formation |
| (57) In a method for controlling the vertical growth of hydraulic fractures in subterranean
formations, the fracturing gradients of adjacent formations are determined and are
used to calculate the fluid density necessary to inhibit the propagation of a hydraulic
fracture from one adjacent formation into the other. A fracturing fluid is then prepared
having the necessary density for inhibiting such hydraulic fracture propagation. |
[0001] This invention is directed to the method of hydraulically fracturing a subterranean formation.
[0002] Hydraulic fracturing techniques have been extensively used for increasing the recovery of hydrocarbons from subterranean formations. These techniques involve injecting a fracturing fluid down a well into contact with the subterranean formation to be fractured. Sufficiently high pressure is applied to the fracturing fluid to initiate and propagate a fracture into the subterranean formation. It is generally considered that at depth the fractures that are formed are vertical fractures. This is because at depth the least principal stress in most formations is in the horizontal plane which produces a preferred vertical fracture orientation. Propping agents are generally entrained in the fracturing fluid and are deposited in the fracture to maintain the fracture open.
[0003] Hydraulic fracturing is widely practiced to increase the production rate from oil and gas wells. Fracturing treatments are usually performed soon after the formation interval to be produced is completed, that is, soon after fluid communication between the well and the reservoir interval is established for the purpose of production or injection. Wells are sometimes fractured for the purpose of stimulating production after significant depletion of the reservoir.
[0004] Hydraulic fracturing is the principal method used for stimulating production from oil and gas wells in low permeability reservoirs. Almost all of such fractures are vertical. It is always desirable, and sometimes necessary, to limit the vertical extent (height) of such fractures to the hydrocarbon-bearing zone of interest while extending the fracture for a substantial horizontal distance. Frequently, the desired horizontal extent (length) is many times the desired height. The desired result can be readily obtained when the interval to be fractured is bounded above and below by beds which inhibit the growth of fractures, such as soft shades. In many other cases the bounding beds are not effective in inhibiting the vertical growth of fractures. This is a major limitation of application of hydraulic fracturing technology. In such cases the resulting fracture grows into the non-productive bounding beds, and some of the valuable fracturing materials are wasted. In cases where permeable beds containing unwanted fluids, such as water, are also penetrated by the fracture a large amount of unwanted fluid is introduced through the fracture into the producing well. In cases where the amount of such unwanted fluid is prohibitive, the well has to be abandoned.
[0005] The present invention seeks to provide a method for controlling the vertical growth of a hydraulic fracture in a subterranean formation located adjacent to another subterranean formation in which the propagation of the fracture is to be inhibited.
[0006] Accordingly, the invention resides in a method of fracturing a subterranean formation that is located adjacent another formation in which fracturing is to be inhibited, comprising the steps of:
a. determining the fracturing gradient in the subterranean formation to be fractured,
b. determining the fracturing gradient in the adjacent formation wherein fracturing is to be inhibited,
c. determining from said fracturing gradients the fracturing fluid density necessary to inhibit the propagation of fracturing into said adjacent formation,
d. selecting a fracturing fluid with the necessary density for inhibiting the propagation of said fracturing into said adjacent formation more than said specified vertical distance, and
e. fracturing said subterranean formation with said fracturing fluid.
[0007] The accompanying drawing illustrates a wellhead penetrating a plurality of subsurface formations, one of which is being fractured by a method according to one example of the present invention.
[0008] Referring to the drawing a well 10 extends from the surface of the earth into subsurface formations 11-15. The well 10 is equipped with casing 16 surrounded by cement 17 which prevents communication in the well outside the casing between the subsurface formations. Communication with a subsurface formation that is hydrocarbon bearing is established by perforations, such as perforations 18, extending into subsurface formation 13 for example. To enhance production from hydrocarbon bearing formation 13 a fracturing treatment is performed through the perforations 18, thereby producing the vertically disposed fracture 19.
[0009] It is a specific feature of the present method to inhibit the growth of fracture 19 and prevent it from penetrating through non-permeable formations, such as 12 and 14, into permeable formations which contain unwanted fluids, such as 11 and 15. Fracture 19 is shown as tending to grow upward into formation 12. Since fracture growth is dominated by the magnitude of the least principal in-situ stress, the stress normal to the fracture plane in formation 12 is not much greater than that in formation 13.
[0010] The least principal stress can be expressed as a fracturing gradient gf, which is the least principal stress Sh divided by the depth Z, that is:
[0012] In order for the fracture to propagate, the fluid pressure in the fracture must exceed Sh. The fluid pressure in the fracture increases linearly with depth, depending on the fluid density, the total pressure at any point being P0 + Pgh, where P is the pressure at a reference point Z0 in the fluid, is the fluid density, g is gravitational acceleration, and h is the vertical distance from the reference point ZO, positive downward. Fluid pressure gradients due to vertical flow in the fracture have been neglected. In practice, it is convenient to express the fluid pressure gradient relative to the gradient gw in pure water, which
[0013] The above information is now used to inhibit the tendency of fracture 19 to grow upward. This is best seen from the following example. Suppose the fracturing gradients gf of formation 13 and gf of formation 12 are determined to be 0.70 psilft (15.83 kPa/m). The fluid pressure in the fracture becomes:
[0014] Letting the reference point ZO be the bottom of formation 13, the fracture is propagated as follows:
[0015] Letting the fracturing fluid be water, then the pressure at any point in the fracture PZ is:
since h is positive downward the pressure P is less than P .
[0016] Thus, the pressure Pfz to fracture at any point Z above Z0 is:
where [h] is the absolute value of h.
[0017] Thus, the fracture will have a strong tendency to propagate upward. In order to inhibit this tendency, we must use a fracturing fluid with a density such that:
p > 0.70 (8.33 ppg)> 13.56 ppg (ppg = pounds per gallon). 0.43 > 1.63 kg/l
[0018] To provide a margin of safety, a fluid weighing 14 to 15 ppg (1.68 to 1.80 kg/1) should be selected.
[0019] From the above example, we see that fractures tend to grow upward for formations with equal fracture gradients which are in the normal range (0.60 to 0.90 psi/ft [13.57 to 20.36 kPa/m]). Fracture gradients usually, but not always, increase with depth.
[0020] A case of downward fracture growth and means for inhibiting, such growth will now be described for an example in which the fracturing gradient in formation 13 is 0.70 psi/ft (15.83 kPa/m) and the gradient in formation 14 is 0.69 psi/ft (15.61 kPa/m). The fracturing pressure at ZO, the bottom of formation 13, is 0.70Zo. The fracturing pressure in bed 14 at any point Zo + h is:
[0021] Just below formation 13 the fracturing pressure in formation 14 is lower than in formation 13 by 0.70 Zo - 0.69 Zo or 0.01 Zo. Thus, the fracture will have a strong tendency to propagate into bed 14. In order to propagate the fracture in formation 13 without continuing to propagate downward in formation 14, PZ must be:
This requires:
[0022] If we let Z0 = 5000 ft. (1524 m), we need 0.69 h > 50 or h> 72.4 ft (22.1 m). But we must also allow for the fracturing fluid head. This requires an additional h, = A h, to balance the fluid head against the fracture gradient difference. This requires:
[0024] The means available for adjusting p /Po are selection of fracturing fluids with different density, selection of different concentrations of propping agent, and selection of propping agents with different density. Any practical combination of fluid, propping agent, and propping agent density may be used.
[0025] The means for determining the fracturing gradient include direct measures of the fracturing pressure and correlations such as these described by Breckels, I. M. and Van Eekelen, H. A. M., "Relationship Between Horizontal Stress and Depth in Sedimentary Basins," Journal of Petroleum Technology, September, 1982, pp. 2191-2199. Pny other suitable means may be used.
[0026] The means of adjustment of fluid density will now be illustrated. In the forgoing examole for an upward growing fracture a fluid with density greater than 13.56 ppg (1.63 kg/1) is needed. This can be achieved by dissolving a suitable amount of sufficiently soluble and dense salt in water, along with gelling agents, etc., for example, sodium bromide. However, it will generally be more economical to achieve the desired density by adding a suitable amount of propping agent to the aqueous fluid. Part of the increase in density can be achieved, if desired, by dissolving inexpensive salts, such as sodium or calcium chloride, in aqueous fluid. In non-aqueous fluids, the same type of procedure can be followed.
[0027] The density of a fluid containing solids pfs, such as propping agents, is given by:
where Pf is the fluid density, Cf is the fraction of unit volume occupied by the fluid, ps is the solid density and Cs is the fraction of unit volume occupied by solid. The density of fracturing fluids and the concentration of solids contained therein is usually expressed in pounds per gallon, ppg:
where Vs is the fraction of propping agent in the slurry. The volume of propping agent in one gallon is:
where Psis the sand grain density in ppg = 8.33 (2.65) = 22.07 (5.83 kg/1). To obtain a water slurry density of 13.56 ppg (1.63 kg/1) we need:
[0028] If a larger increase in density is required than is obtainable by sand, a sintered bauxite or other agent may be used. If a still higher density is required, the density of the base fluid may be increased by addition of a soluble salt.
[0029] To obtain a less dense fracturing fluid a low density liquid, such as diesel oil, may be used. To obtain a still lower density, the aqueous or oil liquid may be mixed with a gas to obtain a stable foam, as is well known to those skilled in the art of hydraulic fracturing. The density of such foams, including propping agents if desired, is calculated in a manner similar to that given above for an aqueous slurry.
[0030] From the forgoing examples, it is seen that control of the vertical growth of fractures is exercised by control of the vertical pressure distribution within the fracturing fluid. If inhibition of upward fracture growth is desired, a dense fracturing fluid is used. If inhibition of downward fracture growth is desired, a light fracture fluid is used. More particularly the fracturing gradients in the formation to be fractured and in the adjacent formation where fracturing is to be inhibited are determined. From this there is determined the fluid density necessary to negate the propagation of the fracture into the formation where fracturing is to be avoided, or the fluid density necessary to minimise penetration of the fracture into the formation to no more than a specified vertical distance. A fracturing fluid is prepared which has more than the minimum density desired if upward propagation is to be inhibited or less than the maximum desired density if downward propagation is to be inhibited, taking into account the amount of propping agent to be used in the fracturing fluid.
1. A method of fracturing a subterranean formation that is located adjacent another
formation in which fracturing is to be inhibited, comprising the steps of:
a. determining the fracturing gradient in the subterranean formation to be fractured,
b. determining the fracturing gradient in the adjacent formation wherein fracturing is to be inhibited,
c. determining from said fracturing gradients the fracturing fluid density necessary to inhibit the propagation of fracturing into said adjacent formation,
d. selecting a fracturing fluid with the necessary density for inhibiting the propagation of said fracturing into said adjacent formation more than said specified vertical distance, and
e. fracturing said subterranean formation with said fracturing fluid.
2. The method of Claim 1 wherein the density of said fracturing fluid is greater than
the minimum fracturing fluid density required for upward fracture growth.
3. The method of Claim 1 wherein the density of said fracturing fluid is less than
the maximum fracturing fluid density required for downward fracture growth.
4. The method of Claim 1 wherein to aid in the inhibition of upward fracture growth
the density of said fracturing fluid is increased by the addition of a granular solid
propping agent, sand, sintered bauxite, and/or a salt soluble in the fluid.
5. The method of Claim 1 wherein to aid in the inhibition of downward fracture growth
the density of said fracturing fluid is decreased by the addition of a low density
oil and/or a gas which forms a stable foam.
6. The method of Claim 1 wherein the fracturing fluid density is determined in accordance
with the following to inhibit upward fracture growth:
where: p = density of fracturing fluid in pounds per gallon,
po = density of water
gf = fracturing pressure gradient of formation being fractured,
gw = fluid pressure gradient in pure water
7. The method of claim 1 wherein the fracturing fluid density is determined in accordance
with the following to inhibit downward fracture growth:
where: p = density of fracturing fluid in pounds per gallon,
po = density of water (8.33 pounds per gallon),
gf = fracturing pressure gradient (psi/ft.) of formation being fractured,
gw = fluid pressure gradient in pure water